The invention relates to the field of microelectronics, and more specifically to the sector for fabricating micro components, especially those intended to be used in radio or microwave applications. More specifically, it relates to electrical resonators that can be incorporated in analogue filters, and which enable the various parameters of such filters to be adjusted.
As is known, electronic circuits used for radio-frequency or microwave applications, in particular such as mobile telephony, comprise filters including oscillating circuits or resonators. Such resonators generally consist of a combination of an inductor and a capacitor.
Under certain conditions, it is necessary to be able to change the parameters of the filter, and in particular its tuning frequency or its bandwidth.
Thus, it has already been proposed to form resonators by combining a capacitor with an inductor, one or other of these components exhibiting parameters which can be changed. Thus, it has been proposed to produce resonators with materials whose properties vary on application of a static magnetic field, such as yttrium iron garnet, commonly called YIG. Such components exhibit the major drawback of a very large footprint.
It has also been proposed to produce components whose properties vary when they are subjected to an electric field, such as ferroelectric materials. In particular, such a component is described in document xe2x80x9cIEEE transactions on microwave theory and techniquesxe2x80x9d, volume 48, number 4, April 2000, pages 525 to 530. Such components have the drawback of requiring relatively high bias voltages, and of exhibiting significant losses.
It has also been proposed to produce variable capacitors based on semiconducting materials. The variation of the capacitance operates on the principle of transfer of charge in the semiconductors. The drawbacks of these devices are significant losses and poor resistance to strong electrical signals.
It has also been proposed to produce variable capacitors by using a bank of elementary capacitors which can be connected in parallel by virtue of switching diodes, making it possible to add the capacitances of each elementary capacitor. This ability has the drawback of providing only a discrete adjustment of the capacitance, and in addition requires relatively high bias voltages.
Generally, all the techniques described above make it possible to produce only components which have relatively mediocre properties in terms of power and of loss.
In documents xe2x80x9cIEEE transactions on microwave theory and techniquesxe2x80x9d, volume 48, number 7, July 2000, pages 1240 to 1246, and xe2x80x9cIEEE transactions on microwave theory and techniquesxe2x80x9d volume 48, number 8, August 2000, pages 1336 to 1343, it has been proposed to produce special resonators using a ribbon conductor arranged in the form of a loop above an earth plane. Such a component, when fed with a radio or microwave signal, operates due to the propagation of this signal between the ribbon conductor and the underlying earth plane. The tuning frequency of such a resonator is therefore directly determined by the length of the ribbon conductor, and more specifically, corresponds to a signal, the half wavelength of which corresponds to the opened-out length of the ribbon.
It will be realized that this type of distributed resonator has many drawbacks. This is because its tuning frequency is directly determined by its geometry, which means that beyond certain frequencies of the order of one gigahertz, such a resonator has dimensions with are incompatible with the production of integrated circuits.
Moreover, from the point of view of its design, such a resonator requires the presence of an earth plane for the propagation of the signal, which therefore gives it a three-dimensional structure which involves some restrictions on the production process.
One problem which the invention proposes to solve is how to adjust the various parameters of the resonator, and in particular its tuning frequency or its bandwidth, and this, over a relatively wide range, while remaining compatible with the footprint constraints of components used in microelectronics.
Another problem which the invention proposes to solve is how to vary the parameters of analogue filters incorporating such resonators.
The invention therefore relates to an elementary electrical resonator. Such a resonator is characterized in that it comprises:
a ribbon conductor forming a flat loop with at least one turn, the ends of which form two parallel segments;
a conducting bridge forming an arch straddling the said segments of the ribbon conductor, the opposing surfaces of the arch and of the said segments forming a capacitor;
and in which a part of the bridge is capable of being displaced with respect to the said segments of the loop under the action of the control signal so as to cause the capacitance of the said capacitor, and therefore the tuning frequency of the resonator, to vary.
In other words, the elementary resonator according to the invention comprises a ribbon forming the inductor, and a conducting bridge which straddles part of the inductor, so as to form a variable capacitor. The combination of this capacitor and of the inductor forms a resonator whose tuning frequency can be changed by varying the capacitance of this capacitor.
In the rest of the description, the ribbon conductor and the conducting bridge can be made from various materials, namely metals or alternatively semiconductors.
The flat loop and the conducting bridge do not require the presence of an earth plane for any signal propagation. In this way such components can be very easily produced, directly on layers of quartz or of silicon or of other types of substrate. These resonators can be integrated into microcomponents specific to filtering functions, or else alternatively they can be produced over an integrated circuit providing other functions.
In practice, the conducting bridge forming the variable capacitor can be deformed by the application of various forces used in the technologies commonly known by the abbreviation xe2x80x9cMEMSxe2x80x9d meaning xe2x80x9cmicroelectromechanical systemsxe2x80x9d. Thus the conducting bridge can be deformed under the action of an electrostatic force using a d.c. voltage applied between the arch and the ribbon conductor. The force which generates the deformation of the arch may also have its origin in a thermal or magnetic phenomenon.
Advantageously in practice, the conducting bridge may be combined with at least one further conducting bridge, arranged in parallel and actuated by a different control signal so as to cause the variable capacitance to vary over a wider range. This therefore amounts to dividing up the total surface forming the capacitor, and causing the elementary capacitor of each bridge to vary independently.
Advantageously in practice, the elementary electrical resonator may in addition comprise:
an additional track, parallel to the segments forming the ends of the loop;
an additional conducting bridge, also forming a variable capacitor, straddling the said additional track and one of the two segments forming the ends of the loop.
In other words, in this configuration, the resonator is combined with an additional capacitor forming a decoupling capacitor.
Thus, the resonator can be used as a filter, when it comprises two connection terminals, that is to say:
a first terminal located on the additional track;
a second terminal located on the segment which is not straddled by the additional conducting bridge.
This filter has an electrical behaviour corresponding to an equivalent circuit comprising, in series, a capacitor and a parallel LC dipole.
By adjusting the additional capacitor, the input impedance of the filter is adjusted, while adjustment of the first variable capacitor makes it possible to tune the resonant frequency of the filter.
The structure of the elementary resonator, (whether or not including the decoupling capacitor as described above) can be used to build filters with several poles, by coupling the various elementary resonators together. It is thus possible to form high-order filters or filters comprising transmission zeros.
In practice, elementary resonators can be coupled by a conducting bridge forming a variable capacitor, which straddles two segments forming the end of a loop of a resonator, these two segments belonging to two different resonators. In other words, two resonators, each including a loop and a conducting bridge, are coupled by one of the ends of their loop, using a bridge forming a variable capacitor. The combination of these two resonators is equivalent to the coupling of two elementary resonators described above by a shared coupling capacitor.
At the level of an equivalent circuit, such an assembly operates as two parallel LC dipoles between which a variable capacitor is connected. Depending on the capacitance of this capacitor which couples the two resonators, it is possible to vary the bandwidth of a filter which includes these two resonators.
The coupling between two elementary resonators may also take place via regions of each ribbon conductor located one facing the other. In other words, each loop has a portion of its length placed side by side with a portion of the other loop, such that the two resonators are coupled by magnetic coupling.
This coupling can be made variable since the regions facing one another can be straddled by an additional conducting bridge which forms a variable capacitor, and which therefore makes it possible to adjust the degree of coupling between the two elementary resonators.
A particular example of a resonator according to the invention may comprise two elementary resonators including a loop and a bridge forming a variable capacitor, and an additional conducting bridge forming an additional variable capacitor, which straddles one of the segments forming one end of the loop of each elementary resonator. In other words, it involves two resonators coupled at the ends of their loop by a shared decoupling capacitor.
In practice, such a resonator may be integrated into a filter which, in addition, comprises two additional tracks, each placed opposite a loop of each elementary resonator, each additional track thus being coupled to the region of the loop opposite, the ends of the two additional tracks forming connection terminals for the filter.
The coupling between the additional tracks and the loops of the elementary resonators can be achieved by two additional conducting bridges forming a variable capacitor, each one straddling an additional track and the region of the loop of the elementary resonator located opposite. Thus, by varying the coupling between the tracks forming the input and the output of the filter and the intermediate resonators, it is possible to vary certain characteristics of the filter such as the input and output impedances, the bandwidth and the central frequency.
Of course, the invention is not limited to filters including two resonators, but covers variants in which the number of resonators is chosen to suit the desired transfer function. It is thus possible to increase the number of resonators, it being thus possible for the total number to be greater than ten.